Likewise, 2011 may be the hottest year on record during La Niña, a periodic cooling of the eastern tropical Pacific.

That's a bad sign, since La Niña years are generally relatively cool, said Steven Running, a professor of ecology at the University of Montana, who was not part of the study team.

So the new finding suggests that La Niña conditions that once produced strong global cooling now only slightly affect the overall temperature trend, Running said by email.

"What does it take now to have a cooling cycle?" he asked. "And what will happen in the next strong El Niño?"

El Niño is a warming of tropical waters in the central and eastern Pacific Ocean. During El Niño years, the warmer currents heat the planet on top of the steady global warming trend caused by human-induced greenhouse gases.

Even that figure might be deceptively optimistic, because much of the sea ice appears to have been thinner than in past years. When sea ice cover was at its smallest in 2011, on September 9, the total Arctic sea ice volume was 8 percent lower than in 2010—previously the lowest on record, the WMO scientists found.

The WMO's Global Atmosphere Watch program also recently released a report concluding that heat-trapping greenhouse gases in the atmosphere had reached a new high—an increase that will only continue, researchers say.

"Our science is solid, and it proves unequivocally that the world is warming and that this warming is due to human activities," WMO Secretary-General Michel Jarraud said in a statement addressing both reports.

Not surprisingly, given the high rates of melting in the Arctic, many Arctic regions were unusually hot. Parts of northern Russia reported springtime temperatures more than 16°F (9°C) above average, the WMO said.

But the most stunning figures may have come from Texas, where daily temperatures averaged 86.7° (30.4°C), in June through August—a staggering 5.4°F (3.0°C) above normal, scientists said.

The Texas statistic is "the highest [such average] ever recorded for any American state," according to the WMO website.

It's difficult to determine exactly how much of the extremes are due to climate change versus normal weather variations, said Richard Alley, a geoscientist at Pennsylvania State University, who was not part of the WMO team.

"The increasing carbon dioxide and other greenhouse gases in the air from our activities do not make 'weather' disappear," he said by email. "But they do 'load the dice' to make hot conditions more likely.

"We haven't made cold snaps, and even record lows, disappear, but data and our physical understanding agree that we're still pushing strongly toward warming."